Patent GB-1053585-A (GB)
IPC: G01N23/207
Abstract: 1,053,585. X-ray analysers. GENERAL ELECTRIC CO. July 30, 1963 [Sept. 5, 1962], No.30255/63. Heading G1A. In an instrument such as an X-ray analyser utilizing radiation to measure a characteristic of a test sample, in which a primary radiation source irradiates the sample and the radiation quanta emanating at a fluctuating rate from the sample are detected and converted into electrical input signals, the signals are integrated in an electrical network having resistive and capacitive components and which has switching means which at the end of a first interval of time and at the end of each of a predetermined number of successive time intervals changes the value of at least one of the said components to increase the time constant of the network to successively reduce fluctuations in the output signal from the network. The figure shows one channel of a multi-channel X-ray analyser in which an X-ray tube 11 is used to irradiate a sample 10 which may be a solid sheet material, a liquid, a powder or a slurry and in which the absorption of the primary X-rays by the sample or as shown the emission of secondary X-rays from the sample are measured in order to determine its characteristics such as thickness, quantity and identity of chemical elements of its constituents, or its atomic lattice structure. The sample may be a continuous stream or individual samples may be consecutively investigated. In the arrangement shown secondary radiation representative of the quantity of an element present in the sample 10 is intercepted by an analysing crystal 14 which diffracts monoenergetic radiation characteristic of that element towards a Geiger Muller proportional counter 15 whose resulting output pulses arrive at a random rate and with variations in their amplitude at an amplifier and a monostable multivibrator 17 which emits a pulse of uniform height and duration for every incoming pulse from the counter 15. The output pulses from the multivibrator are fed to a diode pump circuit which includes capacitors 18, 19 and diodes 20 and 21 and a resistor 22, and the pump converts constant amplitude random pulses to a continuous voltage proportional to the pulse repetition rate. The D.C. voltage appearing across resistor 22 passes through a resistor 23 to a chopper amplifier 24, 25 which is stabilized by a feedback resistor which also forms part of a time-constant circuit with a capacitor 30. At the output terminal 27 of the amplifier 24, 25 an analogue voltage appears representative of the quantity of the element present in the sample and this may be read on a voltmeter 28 or as a trace 12 on a chart recorder 13. The timeconstant of the combination of capacitor 30 and resistor 26 is about 5 seconds. With such a time constant, the random nature of the pulses reaching the amplifier 17 results in large fluctuations at the bottom end of the trace 12. These fluctuations may be reduced by increasing the time constant but at longer time constants a long period of time must elapse before the apparatus stabilizes sufficiently to enable a reasonable reading to be made from the trace 12. This is overcome by the provision of a bank of capacitors 40, 41, 42 which are added in parallel one by one to the capacitor 30 under the action of switches 43, 44, 45 of relays 46, 47, 48 the operations of which are controlled through cams 59, 60, 61 by a motor 52. When the capacitors 40, 41, 42 are disconnected from parallel operation with the capacitor 30 by the opening of switches 43, 44, 45, they are connected by switches 49, 50, 51 of the relays 46, 47, 48 to the output of 27 the amplifier 24, 25 so that they will always be charged to a suitable value such that when they are reconnected to the capacitor 30 stabilization of the apparatus occurs quickly without the necessity of waiting for the longer times to elapse. In a further embodiment the resistive portion of the timeconstant circuit may be altered by successive stops Figs. 33 (not shown). For light elements, randomness of emitted photons is pronounced, and because of the resulting low number of pulses arriving at the capacitor over a short time interval, a short counting interval might yield a count above or below the mean value. Thus the time constant of the circuit is initially made short to enable the best possible accuracy even though the statistical fluctuations are evident on the chart trace 12 and then the time constant is lengthened until a practical degree of precision is reached. A standard sample 31 may be provided which is inserted in the X-ray beam by a crank 32 and motor 35. The Geiger- Muller detector 15 may be replaced by a scintillator.